Wireless communication systems that were providing voice-based services have evolved to broadband wireless communication systems that are capable of providing packet data services based on high quality and high speed, such as: Long Term Evolution (LTE), High Speed Packet Access (HSPA) defined in 3rd Generation Partnership Project (3GPP); Ultra Mobile Broadband (UMB), High Rate Packet Data (HRPD) defined in 3rd Generation Partnership Project 2 (3GPP2); the communication standard IEEE 802.16e; etc.
The LTE system, as a typical example of the broadband wireless communication systems, employs Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Single Carrier-Frequency Division Multiple Access (SC-FDMA) in the uplink. The Multiple Access performs allocation and management of time-frequency resources to carry data and control information according to users, so as not to overlap with each other, i.e., so as to achieve orthogonality between them, thereby distinguishing data or control information between respective users.
FIG. 1 is a diagram showing the basic structure of a radio resource area on the time-frequency domain, transmitting data or control information through an uplink of an LTE system.
In LTE systems, uplink (UL) refers to a radio link through which UE transmits data or control signals to evolved Node B (eNB) (base station) and downlink (DL) refers to a radio link through which eNB transmits data or controls signals to UE.
As shown in FIG. 1, the horizontal and vertical axes represent the time and frequency domains, respectively. The minimum unit of transmission on the time domain is an SC-FDMA symbol. Nsymb SC-FDMA symbols (N represents the number of symbols), indicated by the reference number 102, form one slot 106. Two slots 106 form one subframe 105. 10 subframes 105 form one radio frame 107. The slot has a length of 0.5 ms. The subframe has a length of 1.0 ms. The radio frame has a length of 10 ms. The minimum unit of transmission on the frequency domain is a subcarrier.
The basic unit of resource on the time-frequency domain is a Resource Element (RE) 112 and is represented by an SC-FDMA symbol index and a subcarrier index. The Resource Block (RB) 108 (or Physical Resource Block (PRB)) is defined as successive Nsymb SC-FDMA symbols 102 on the time domain and successive NRBSC subcarriers 110 on the frequency domain. Therefore, one RB 108 includes REs of Nsymb×NRBSC, denoted as Nsymb×NRBSC REs 112. In general, the minimum unit of data is an RB 108 and the system transmission bandwidth forms RBs of NRB in total, denoted as NRB RB 108. The overall system transmission bandwidth is subcarriers of NRB×NRBSC in total, denoted as NRB×NRBSC subcarriers 104. Generally, in LTE systems, Nsymb=7 and NRBSC=12.
The LTE system employs a Hybrid Automatic Repeat reQuest (HARQ) scheme for retransmitting data, which has failed in decoding in the initial transmission, via the physical layer. HARQ is a scheme that allows a receiver to transmit, when not correctly decoding data from a transmitter, information (NACK) indicating the decoding failure to the transmitter so that the transmitter can perform re-transmission of the data from the physical layer. The receiver combines the data re-transmitted from the transmitter with the existing data for which decoding has failed, thereby increasing the capability of data reception. When correctly decoding data, the receiver transmits information (ACK) indicating the success of decoding to the transmitter so that the transmitter can perform transmission of new data.
In broadband wireless communication systems, one of the important factors in providing high transmission rate wireless data services is the ability to support scalable bandwidths. For example, LTE systems are capable of supporting various bandwidths, such as 20/15/10/5/3/1.4 MHz, etc. Therefore, service operators are capable of selecting a particular one of the various bandwidths and providing services via the bandwidth. There are various types of user equipment (UE) devices that are capable of supporting bandwidths from a minimum of 1.4 MHz to a maximum of 20 MHz.
FIG. 2 is a diagram showing the structure of an LTE-A system supporting carrier aggregation.
As shown in FIG. 2, eNB (base station) 202 supports the aggregation of two component carriers, CC #1 and CC #2. CC #1 has a frequency f1 and CC #2 has a frequency f2 that differs from f1. CC #1 and CC #2 are included in the same eNB 202. The eNB 102 provides coverage 104 and 106 corresponding to the component carrier CC #1 and CC #2, respectively. The LTE-A system capable of supporting carrier aggregation performs transmission of data and transmission of control information related to the transmission of data, according to component carriers, respectively. The configuration shown in FIG. 2 may also be applied to the aggregation of uplink carriers in the same way as the aggregation of downlink carriers.
The carrier aggregation system divides component carriers into Primary Cell (Pcell) and Secondary Cell (Scell) and manages them. Pcell refers to a cell that provides the basic radio resources to UE and serves as a standard cell allowing UE to perform operations such as the initial access, a handover, etc. Pcell includes a downlink primary frequency (or Primary Component Carrier (PCC)) and an uplink primary frequency. Scell refers to a cell that provides additional radio resources to UE along with Pcell. Scell includes a downlink secondary frequency (or Secondary Component Carrier (SCC)) and an uplink secondary frequency. In the present disclosure, unless otherwise indicated, the terms ‘cell’ and ‘component carrier’ will be used interchangeably with each other.
The Frequency Division Duplex (FDD) scheme employs different frequencies for downlink and uplink. In contrast, the Time Division Duplex (TDD) scheme employs the same frequency for downlink and uplink but performs transmission and reception of uplink/downlink signals at different times. The LTE TDD scheme transmits uplink or downlink signals at different times according to subframes. Therefore, on the time domain, according to traffic load of uplink and downlink, the LTE TDD is capable of: dividing subframes to uplink/downlink equally and managing them; or assigning more subframes to either downlink or uplink and managing them.
TABLE 1Uplink-downlinkSubframe numberconfiguration01234567890DSUUUDSUUU1DSUUDDSUUD2DSUDDDSUDD3DSUUUDDDDD4DSUUDDDDDD5DSUDDDDDDD6DSUUUDSUUD
Table 1 shows the TDD uplink-downlink configuration defined as in LTE. In table 1, ‘D’ denotes a subframe configured for downlink transmission, ‘U’ denotes a subframe configured for uplink transmission, and ‘S’ represents a Special subframe including a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP), and an Uplink Pilot Time Slot (UpPTS).
FIG. 3 is a diagram showing the structure of a special subframe for an LTE TDD system.
Referring to FIG. 3, DwPTS 301 is used to transmit control information via downlink like a general subframe. When DwPTS 301 has a sufficient length according to configuration states of a special subframe, it can be used to transmit downlink data. GP 302 is a section for accepting the transition of transmission signals from a downlink to an uplink, and its length is determined according to the network settings, etc. UpPTS 303 contains one or two SC-FDMA symbols and is used to transmit Sounding Reference Signal (SRS) of UE which eNB needs to estimate an uplink channel state or a random access preamble of UE to perform random access.
The special subframe has a length of 1 ms like the general subframe. According to the settings of eNB, DwPTS 301 includes 3 to 12 OFDM symbols and UpPTS 303 includes 1 or 2 SC-FDMA symbols. GP 302 has a time interval obtained by subtracting the length of DwPTS 301 and UpPTS 303 from the overall length of the special subframe, 1 ms.
As described in table 1, the special subframe may be set to subframe #1 or subframe #6 according to the TDD uplink-downlink configuration.
For example, for TDD uplink-downlink configuration #6, subframe #0, #5, and #9 may transmit downlink data and control information, and subframe #2, #3, #4, #7, and #8 may transmit uplink data and control information. Subframe #1 and #6 corresponding to the special subframe may transmit downlink control information and further downlink data according to conditions. Sounding Reference Signal (SRS) or RACH may be transmitted via the uplink.
eNB estimates an uplink channel state from an SRS transmitted from UE. In general, an SRS may be located in the last SC-FDMA symbol of a subframe. In an LTE system using a TDD scheme, the UpPTS section of the special subframe may transmit SRS over a maximum of two SC-FDMA symbols. eNB may determine a subframe, available to transmit an SRS, and an SC-FDMA symbol in the UpPTS, available to transmit an SRS, and inform UE of the settings via signaling.
A conventional LTE-A system configured to support carrier aggregation has a limitation to apply the same duplex scheme to individual component carriers. That is, it aggregates component carriers using the FDD scheme to each other or component carriers using the TDD scheme to each other.
In order to perform carrier aggregation using duplex schemes that differs from each other according to component carriers, the present invention provides a method for UE to transmit an SRS via a special subframe.